Federal authorities have joined an expanding investigation after Stockton, California, police linked two more shootings to a series of killings that have shaken the city.
Authorities had previously said the fatal shootings of five men between July 8 and Sept. 27 may be "interconnected."
A combination of video surveillance or ballistics led authorities to conclude that those slayings and the two additional shootings, including one in Oakland, are connected.
"Last night was the new link that expanded to Oakland," Stockton police spokesperson Joe Silva said. "Our officers are going back to more old cases we have, and they're also, obviously, reaching out to other regional law enforcement agencies to see if they've had any type of similar cases."
In the first case, a Latino man was fatally shot just before 4:20 a.m. on April 10, 2021, in Oakland. Days later, a 46-year-old Black woman was shot at 3:20 a.m. on April 16 in the area of Park and Union streets in Stockton.
The Alameda County Coroner's Bureau on Tuesday identified the victim in the Oakland shooting as Juan Vasquez Serrano, 39, of Oakland.
The woman in the Stockton shooting survived her injuries, according to a statement posted on the department's Facebook page Monday night. Her name was not released.
Silva told NBC News on Tuesday that investigators with the FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and the San Joaquin County Sheriff's Office are part of the investigation.
The additional personnel are with task forces that routinely work with the department, Silva said.
Stockton is a city of about 322,000 residents, according to the U.S. Census Bureau.
The city saw 39 homicides in 2021 and 43 so far this year, Silva said.
He said a commonality in all seven shootings appears to be the victims were ambushed under the cover of darkness.
"Our victims are alone. They're in dark areas. They're occurring during evening hours, early-morning hours," Silva said.
A $95,000 reward is being offered for information that leads to an arrest in the series of killings, all of which happened in areas with few security cameras, Stockton Police Chief Stanley McFadden said Friday.
"It wasn't a robbery," he said. "Items aren't being stolen. They're not talking about any gang activity in the area or anything. It's just element of surprise."
An unidentified person recorded on grainy security video could be a suspect or a witness, police said, and other people may be involved in the attacks.
"We don't know if there is one individual or if there is a series of individuals that are responsible - responsible for these homicides," Stockton Mayor Kevin Lincoln told CNN on Tuesday. "These are by definition a series of killings in Stockton, California. We know it expanded outside the city and we're taking it very seriously."
The San Joaquin County medical examiner's office on Monday identified the five victims of the July through September shootings as: Paul Alexander Yaw, 35, of Stockton, who died July 8; Salvador Debudey Jr., 43, of Stockton, who died Aug. 11; Jonathan Hernandez Rodriguez, 21, of Stockton, who died Aug. 30; Juan Cruz, 52, of Stockton, who died Sept. 21; and Lawrence Lopez Sr., 54, of Stockton, who died Sept. 27.
Family members of Yaw and Lopez said that both men had fallen into homelessness.
Analydia Lopez, Debudey's wife, said she was heartbroken by the loss of her husband of 12 years.
"To be honest with you, a part of me died with him that day," Lopez said. "It's been hard. It's been really, really hard."
This article was originally published on NBCNews.com